Can a lack of backpressure cause knock? Here's why I ask.... In my late model Nip job, about 2 months ago, my knock sensor meter started showing that it was detecting a slight knock. I had no idea what was causing it. It didn't go away with 101 octane fuel, it didn't go away when the outside temps are 55 degrees at night. I have been stumped. When I first noticed the problem, my catalytic converter was in place. After my emissions test, I removed the cat in favor of the straight pipe that had been in there for years. It seemed to knock worse, according to the gauge, and one night, while climbing through the mountains on I-17, I actually heard it knock. Might not sound like much, but all the Nip gurus were telling me that when you can hear it, it is VERY bad. So, I finally noticed that there was a huge hole in the flex section of the exhaust. I never noticed, because it has a turbo, and that must have muffled the noise. I replaced that flex section and it seems to have gotten better, little or no movement of the knock sensor needle, and ONLY when I really try to make it happen.
My line of thinking might be way off, but this is the only way I could explain it... If the hole in the exhaust was behind the turbo, the pressure difference in front to behind the turbo is bigger, making it more efficient. ( if the waste gate does not compensate for it ) Also, if the burnt gasses can escape the combustion chamber easyer, more combustable ones can enter, raising the pressure on combustion.
A Knock sensor doesn't sense "Knocking", it senses preignition. The computer then adjusts timing and mixture to compensate so you don't melt a piston. The hole might have caused the engine to run leaner than the computer could compensate for so you ended up with some pinging. (Everything is back to normal?) Seems everything did its job as designed! Carry on. Bill
The Jalopy Journal
